1. Field of the Invention
The present invention relates to a retardation film and a liquid crystal display device using the retardation film, and, in particular, relates to a retardation film which is suitably applicable to a ½λ plate, a ¼λ plate, a protection film, an anti-reflection film, and the like, and to a liquid crystal display device using the retardation film.
2. Related Background Art
In liquid crystal display devices, such as liquid crystal display (LCD), a retardation film having a controlled optical anisotropy is used for the purpose of optical compensation. Conventionally, materials like polycarbonate and cyclic polyolefin, which have positive birefringences, have been mainly employed.
Under such a circumstance, norbornene-based ring-opening polymers based on norbornene derivatives, which are especially highly reactive among cyclic polyolefins, as precursors, have been developed. For example, Japanese Unexamined Patent Application Publication No. 2003-255102 (Document 1), Japanese Unexamined Patent Application Publication No. 2004-176051 (Document 2), and Japanese Unexamined Patent Application Publication No. 2004-323489 (Document 3) each disclose an optical film made of a specific norbornene-based ring-opening polymer. However, such conventional norbornene-based ring-opening polymers disclosed in Documents 1 to 3 do not have a sufficient adhesiveness to other materials, and do not sufficiently achieve negative birefringences which are expected to be utilized for a reverse wavelength dispersion film and the like.
Meanwhile, U.S. Pat. No. 5,612,801 (Document 4) discloses a so-called negative A-plate retardation film which exhibits optical characteristics unique even among negative birefringence characteristics. However, such conventional materials like polystyrene and polymethyl methacrylate which have negative birefringences do not have a sufficiently high heat resistance, and it has been difficult to obtain a negative A-plate retardation film made of any of these materials.
In addition, various studies have been conducted on retardation films (broadband retardation plate: reverse wavelength dispersion film) providing a ¼ wavelength retardation in a wide wavelength range of the entire visible light region (400 to 800 nm), which is required for the applications of LCDs, pickups for optical disks, and the like. For example, Japanese Unexamined Patent Application Publication No. Hei 10-68816 (Document 5) discloses a retardation film in which two retardation films are pasted and laminated in a substantially orthogonal direction. Furthermore, Japanese Patent Unexamined Application Publication 2002-48919 (Document 6) discloses a retardation film which is obtained by polycondensation of two kinds of monomers, and which exhibits positive or negative inherent birefringence. Furthermore, Japanese Unexamined Patent Application Publication 2001-194527 (Document 7) discloses a retardation film which is obtained by alloying of polymers exhibiting positive and negative inherent birefringences. Japanese Unexamined Patent Application Publication 2005-36201 (Document 8) discloses a retardation film which is obtained by ring-opening (co)polymerization of two kinds of norbornene-based monomers exhibiting positive or negative inherent birefringence after the polymerization.
However, in the retardation film disclosed in Document 5, it is necessary that two films be pasted with each other using an adhesive agent in the production. Accordingly, there have been problems, in productivity, that the yield is lowered due to the complicated production process, and that the cost is high. Meantime, in the conventional retardation films disclosed in Documents 6 to 8, white turbidity is frequently formed by phase separation in blending during the production because polymers are difficult to be mixed with each other. Accordingly, it is difficult to use the obtained retardation film for optical applications. Furthermore, since monomers exhibiting negative birefringence which are used in the production of these retardation films have a special structure, there is a problem that the production is difficult and costly.